Sound has no fixed speed — it borrows one from the air. In dry air the rule is $v = 331.3 + 0.606\,T$ — the speed $v$ in m/s, and $T$ the air temperature in °C. Careful with that letter: last lesson $T$ was a period in seconds, and here it is a temperature. The formula tells you which. So 331.3 m/s on a freezing day, and about six-tenths of a metre per second more for every degree above. Room temperature lands near 343 m/s. Warmer molecules pass the message faster — that is the whole story.

Then the classic exam wound: the echo. A shout that bounces off a cliff crosses the gap TWICE — out and back — so the wall stands at $d = \dfrac{v\,t}{2}$, never $v\,t$, with $d$ the one-way distance in metres and $t$ the whole out-and-back time in seconds. Thunder is the opposite trap: the lightning's flash arrives essentially instantly, the rumble makes the trip once, and no ÷2 is invited. Read the journey before you divide.